The invention relates generally to biomedical implant telemetry systems for analog physiological data and more particularly to intracardiac waveform transmission from an implanted cardiac pacer.
For many years fully implanted tissue stimulators have been used to treat cardiac and nervous disorders. Microelectronic circuits inside a hermetically sealed implanted case generate electrical impulses according to a prescribed set of parameters. By far the most common species of this type of implant is the cardiac pacer. A sealed battery powered pulse generator is connected to an insulated electrical conductor which passes into and through the vascular system and terminates in an electrode which is attached inside the heart, for example, to the bottom of the right ventricle. Electrical impulses generated by the sealed circuitry are applied via this lead, ordinarily using a metal case of the pulse generator as a ground electrode. If the stimulation pulse is correctly timed and exceeds a so called capture threshold, the ventricle will contract in response to the electrical stimulation. The same lead is employed as an electrical sensor to detect naturally generated electrical impulses which characterize spontaneous cardiac activity to inhibit artificial stimulation to avoid competition with the natural heart rhythm.
It is now commonplace to prescribe changes to the stimulation pulse parameters and other criteria by externally transmitting coded signals to the implanted circuitry. The first commercially successful type of inbound data transmission used electromagnetic impulse programming to rapidly actuate a tiny reed switch connected to a counter chain. Pulse width modulation systems have evolved based on improved electromagnetic impulse programming as well as RF signalling.
Outbound telemetry systems have recently been introduced to allow two-way communication between the external programmer and the implant. One of the most severe restrictions on outbound telemetry systems is power consumption. Battery operated pacers are designed to remain implanted for five to ten years. A conventional transmitter inserted in the pacer would consume too much power if used for any significant length of time. To overcome this limitation, resonant reflected signal transponder-like systems have been proposed in which the carrier frequency is supplied externally and modulated internally by the implant. Impedance modulated resonant transponder circuit systems are shown in U.S. Pat. No. 4,361,153 assigned to the assignee of the present invention and incorporated herein by reference. U.S. Pat. No. 4,361,153 discloses an outbound telemetry system which allows transmission of information from the implanted device while consuming a minimum amount of power. This telemetry system includes a resonant impedance modulated transponder in the implant which modulates the phase of the carrier in accordance with a pulse width modulated binary signal representative of the condition of the parameter registers in the implant. In this way, when interrogated, the pacer can inform the physician of the current programmed values of the various programmable parameters. U.S. Pat. No. 4,223,679 to Schulman purports to disclose an impedance reflecting resonant circuit apparently relying on frequency modulation of the carrier.
In addition to digitally stored parameter data, stored programs (software) and the like, it would be desirable to have a means for transmitting out of the pacer certain measured or sensed variables which are analog in nature. Two of the most interesting variable quantities are battery voltage and electrical amplitude of natural activity on the cardiac lead. Absent stimulation, the lead acts as an electrical pickup and electrical signals of varying amplitude appear on the lead. If the lead is attached to the inside of the heart, the resulting signal is known as an intracardiac electrogram or ICEG signal. It is desirable to obtain an ICEG since it is a different, and in some cases, a better means of studying the electrophysiology of the heart. Electrophysiological studies of the heart are extremely important in diagnosing and treating certain arrhythmias. The ICEG cannot be duplicated externally by a conventional EKG. It is, however, extremely difficult to reliably transmit the ICEG signal given the battery power constraints of the pacer. Analog to digital conversion of the ICEG signal would be ideal given the proper sampling rate because of the inherent fidelity of the signal. However, A/D converters consume excessive power. Analog transmission on the other hand presents the problem of base line calibration and possible distortion which would not necessarily be present in the A/D conversion option.
One of the items of interest for an ICEG transmitter would be the ability to inspect the ICEG signal immediately following the application of an artificial stimulation pulse to see whether capture had occurred. This aim is frustrated by the retention on the lead of a decaying charge after the stimulation pulse which can mask natural electrocardiac activity.